1. Field of the Invention
The present invention relates to ferromagnetic fine particles of .epsilon.' iron carbide and a method for producing the same. More particularly, the present invention relates to ferromagnetic fine particles of .epsilon.' iron carbide having a composition of .epsilon.'-Fe.sub.2.2 C and a specific particle shape and size, and a method for producing such ferromagnetic fine particles of iron carbide.
2. Description of the Related Art
As magnetic recording materials, magnetic metal oxides or metals such as acicular .gamma.-Fe.sub.2 O.sub.3, CrO.sub.2, .alpha.-Fe and the like are used.
Since .gamma.-Fe.sub.2 O.sub.3 has small coercive force, shape anisotropy has to be induced through increase of its acicular ratio or crystalline anisotropy has to be imparted through coating of particle surfaces with cobalt oxide.
Although CrO.sub.2 has good magnetism and particle shape (acicular shape), its Curie temperature is low so that its characteristics considerably vary with temperature. Therefore, a magnetic recording medium of CrO.sub.2 as the magnetic material should be used under very limited conditions. Further, a magnetic tape containing CrO.sub.2 particles causes heavy wear of a magnetic head, and formed Cr(IV) pollutes the environment.
Since .alpha.-Fe powder has a higher coercive force and at least twice larger value of magnetization than .gamma.-Fe.sub.2 O.sub.3, it will satisfy requirements for high density recording. However, since the metal powder has high chemical reactivity, its production process is somewhat complicated and the powder should be carefully treated. After the particles are formed into the tape, deterioration of magnetic properties due to oxidation should be prevented and abrasion because of softness of metal should be prevented. Further, a specific surface area of .alpha.-Fe acicular particles to be used in an 8 mm video tape should be increased. However, according to the conventional method, increase of the specific surface area of .alpha.-Fe powder will lead to decrease of magnetization, and at a specific surface area of 80 to 100 m.sup.2 /g, .alpha.-Fe powder tends to be converted to simple iron oxide. Then, such defects should be removed (see Tatsuji KITAMOTO, "Kagaku Sosetsu" (Survey on Chemistry) No. 48, "Fine Grain , its Chemistry and Applications" edited by the Chemical Society of Japan, Gakkai-shuppan Center (1985)).
With respect to the acicular magnetic particles for magnetic recording which utilizes shape magnetic anisotropy and is said to have been firstly proposed by M. Camras (cf. Japanese Patent Publication No. 776/1951), the acicular particles suffer from the above described defects or print through. At present, decrease of the particle size has its limit.
Iron carbide for magnetic recording includes spherical iron carbide (cf. U.S. Pat. No. 3,572,993) and iron carbide comprising .chi.-Fe.sub.5 C.sub.2 (cf. Japanese Kokai Patent Publication No. 111923/1986). Both of them comprise .chi.-Fe.sub.5 C.sub.2 as the main component and are a mixture of said main component and other iron carbide such as .epsilon.-Fe.sub.2 C, .epsilon.'-Fe.sub.2.2 C, .theta.-Fe.sub.3 C, etc. Then, they are expressed by a composition formula: Fe.sub.x C wherein x is a number not smaller than 2 and not larger than 3.
The spherical iron carbide is of fine particles having an average particle size of 0.005 to 0.1 .mu.m and a coercive force of 500 to 700 Oe, which is smaller than the reported coercive force of iron carbide (J. Phys. Chem., Vol. 64, 1720 (1960)). It is produced by reacting a steamed aggregate of iron carbonyl with carbon monoxide or a mixture of carbon monoxide and hydrogen. It is dangerous to use iron carbonyl which easily ignites and explodes in the air.
The iron carbide comprising .chi.-Fe.sub.5 C.sub.2 partly contains fine particles having an average particle size of 0.1 to 5 .mu.m and an average axial ratio of 1.0 to 3.0 or more than 3. However, most of the particles cannot be fine grains. It has magnetic characteristics such as saturation magnetization of 50 to 90 emu/g and coercive force of 400 to 900 Oe. In addition, the magnetic characteristics largely scatter.
The iron carbide comprising .chi.-Fe.sub.5 C.sub.2 can be produced by reducing iron oxyhydroxide or iron oxide with hydrogen and reacting the reduced product with a reducing and carbonizing agent containing carbon and optionally hydrogen. Since the reduction with hydrogen is carried out in a temperature range from 200.degree. to 700.degree. C., at higher temperature, the particles tend to be sintered together. Further, since the carbonization temperature is comparatively high, such as 250.degree. to 400.degree. C., the particle size distribution is broadened or carbon is deposited at a high deposition rate, which makes it difficult to control the composition of the final product of iron carbide. Thereby, the magnetic characteristics scatter, and the deposited carbon deteriorates the magnetic characteristics. In addition, a fair amount of Fe.sub.3 O.sub.4 and carbon which further makes it difficult to control the composition remain in the product.
As described above, many processes for producing iron carbide have been proposed. However, by the conventional processes, only the iron carbide comprising .chi.'-Fe.sub.5 C.sub.2 as the main component can be obtained, and by-products such as .epsilon.'-Fe.sub.2 C, .epsilon.'-Fe.sub.2.2 C, .theta.-Fe.sub.3 C and the like are present as mixed phases in .epsilon.'-Fe.sub.5 C.sub.2, which by-products cannot be isolated.
According to J. Amer. Chem. Soc., vol. 81, 1576 (1959), the value of saturation magnetization of the carbide phase is larger on the order of .epsilon.'-Fe.sub.2.2 C&gt;.theta.-Fe.sub.3 C&gt;.chi.-Fe.sub.5 C.sub.2. Therefore, .epsilon.' iron carbide is highly promising as a magnetic recording element.